A Sustainable Dual Cross-Linked Cellulose Hydrogel Electrolyte for High-Performance Zinc-Metal Batteries

Highlights A sustainable dual cross-linked cellulose hydrogel with excellent mechanical strength was fabricated from aqueous alkali hydroxide/urea solution using a sequential chemical and physical cross-linking strategy. The hydrogel electrolyte effectively suppresses dendrites growth and side reactions to achieve a stable Zn anode (over 2000 h for Zn||Zn cell), which are proved by a multi-perspective and in-depth mechanism investigation. The hydrogel electrolyte is easily accessible and biodegradable, making the zinc batteries attractive in terms of scalability and sustainability. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01329-0.

where εf and ε0 are the initial strain and fracture strain, respectively.The elastic/compressive modulus was calculated according to the initial linear slope (0.1−1% strain) of the stress−strain curve.For recovery experiment, the hydrogel was initially compressed to a predetermined strain (50%) and then unloaded at the same speed (10 mm min -1 ).

S1.2 Electrochemical Measurements
CR2032-type coin cells were assembled in air atmosphere for most electrochemical measurements.The 0.70-mm thick DCZ-gel film was cut into disks (diameter of 16 mm) to be used as the hydrogel electrolyte.When the liquid electrolyte (i.e., aqueous solution of 1 M Zn(OTf)2) was used, a glass fiber membrane (GF/D, Whatman, diameter of 16 mm) was employed as the separator.The Zn||Zn, Zn||Cu, Zn||PANI coin cells were fabricated by using Zn foils (thickness of 200 μm and purity of 99.99%), Cu foils (thickness of 20 μm), and the PANI/CC cathode with the same diameter of 10 mm, as well as.
All the electrochemical measurements were operated at 25 °C unless otherwise specified.The charge−discharge measurements of the above cells were conducted on a battery testing system (CT2001A, LAND, China).The charge cutoff voltage was 0.5 V for the Zn||Cu cells, and the voltage window was 0.5−1.5 V for Zn||PANI.The cyclic voltammetry (CV), linear scan voltammetry (LSV), Nano-Micro Letters S2/S14 chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) were carried out on an electrochemical workstation (1470E, Solartron analytical, USA).For EIS tests, the frequency range was from 100 kHz to 0.1 Hz and the voltage amplitude was 10 mV.
Tafel tests were conducted using a three-electrode system made up of Zn, Pt, and Hg/Hg2SO4 electrodes as working, counter, and reference electrodes, respectively.For testing the DCZ-gel electrolyte in the three-electrode system, the Zn electrode was wrapped with the hydrogel with a thickness of ∼0.70 mm and then immersed in the liquid electrolyte (1 M Zn(OTf)2).LSV measurements were conducted using the same device, and aqueous solutions of 1 M NaOTf and 1 M Zn(OTf)2 were used for cathodic and anodic scan, respectively.
The ionic conductivities (σ) were obtained from EIS measurements and calculated by the following equation: where l, R, and A are the thickness, bulk resistance, and area of the hydrogel electrolyte, respectively.The transference numbers of Zn 2+ ion (tZn 2+ ) were obtained from the EIS measurements of the Zn||Zn symmetrical cells before and after a polarization (CA test) under 20 mV for 1000 s, and calculated by the following equation: where I0 and R0 are the initial current and resistance before polarization, Is and Rs are the steadystate current and resistance after polarization, and ΔV is the polarization voltage (20 mV for a Zn electrode).The desolvation process of Zn 2+ is usually the rate-limiting step of Zn deposition, which can be expressed by the activation energy (Ea) in the Arrhenius equation: where Rct is the charge transfer resistance, A is the frequency factor, R is the gas constant, and T is the absolute temperature.

S1.3 In situ Observation of Zn Dendrite Growth
A homemade transparent Zn||Zn cell was designed for in situ observation of the Zn plating/stripping processes with the liquid and DCZ-gel electrolytes.Two Zn foils (10 mm × 50 mm, thickness of 100 μm) were fixed in a cuvette by insulating tape, and separated by the electrolyte with a thickness of 1 mm.The cells were tested using a chronopotentiometry (CP) method on an electrochemical workstation (CHI760E, Shanghai Chenhua, China) at a current density of 5 mA cm −2 , and simultaneously observed under an optical microscope.

S1.4 DFT Calculations
All DFT calculations were performed using Gaussian 16 software [S1].The B3LYP/6-31g* level was chosen to compute the geometrical optimizations, electron configurations, and Gibbs free energies of all species [S1].Vibrational frequency calculations were carried out at the same level in order to verify the optimized structures at the local minimum.To calculate the weak interaction between two molecules, the basis set superposition error (BSSE) was used to eliminate the basic function of the two molecules overlapping in the complex system [S2].In addition, the solvation effect was considered by employing SMD methods [S3], and water was used as the solvent.The binding energy (Eb) of Zn 2+ ion with a specified molecule (M) was calculated by the following equation: where M is 6H2O or the simplified model molecule of cellulose chains, and E(Zn 2+ -M), E(Zn 2+ ), E(B), and E(BSSE) are the Gibbs free energies of Zn 2+ -M complex, Zn 2+ , M, and BSSE, respectively.

Finite
element simulation was conducted to analyze the electric field and ionic concentration distribution in the liquid and DCZ-gel electrolyte.The models were simplified to a unit with a height of 18 μm and a width of 24 μm.The protrusions on the Zn surface were represented by semi ellipses with a long axis of 2 μm and a short axis of 1 μm.According to the structural characteristics of DCZ-gel electrolyte, its model with channels were constructed.The simulations were performed by COMSOL Multiphysics based on solving the Nernst-Planck equation.The ion migration and diffusion behavior driven by the established electric fields were considered during the process.The boundary conditions of cathode and anode were set as experimentally measured voltage hysteresis and a constant of 0 V, respectively.For the simulation of hydrogen evolution, the exchange current density parameter was introduced to solve the Butler-Volmer equation.The exchange current density of hydrogen evolution was obtained by experimental measurement and calculation.

Fig. S2 aFig. S4 aFig. S6 a
Fig. S2 a The full XPS spectra, b corresponding peak fitting of C1s, and c solid-state 13 C NMR spectra of the Cel-gel and DCH-gel

Table S1
The electrochemical performances comparison between the DCZ-gel electrolyte and other reported polysaccharide-based hydrogel electrolytes for ARZBs